Electrostatic Adsorption and Ion Exchange

The same argument has been advanced more quantitatively for the respecia-tion of gold chlorides adsorbed onto alumina [27]. The adsorbed Au complexes contain less chloride than those in solution at the same bulk pH, however, when the data for the adsorbed species was replotted at the estimated surface pH, the speciation agreed reasonably well with the bulk-solution species. 

An electrostatic mechanism with respeciation provides a reasonable explanation for the formation of the hydroxyl-bridged Cu dimer reported in the literature [28] in preparations with Cu ammines the tetraammine complex is electrostatically attracted to the silica surface, and when it approaches the surface it respeciates according to the local pH. The copper dimer species is more compact than monomeric Cu ammines, and so attains a higher Cu surface density. 

The uptake-pH surveys of a number of other metals (Ru(II), Ru(III), Co, and Ni beyond Cu, Pt and Pd) over amorphous and mesoporous silica have recently been reported [24]. It is beheved in all cases that adsorption is all or mostly electrostatic, with minor deviations due to metal omplex respeciation (or in the case of Co, phyllosihcate formation at the highest pH, as will be discussed later). When synthesized at the optimal pH (10-11), very well dispersed particles form (with the exception once again of Co since Co ammines reduce on silica only at temperatures in excess of 700 C, where sintering cannot be prevented). DI, on the other hand, yielded much larger particles for all metals over both types of supports. 

The deposition of metal ammines complexes onto silica at high pH is often referred to as ion exchange. In several recent papers we have suggested that electrostatic adsorption is a much better description of this phenomenon [14, 

28] since the uptake (i) is very pH dependent yet produces no pH shifts that differ from metal-free control experiments and (ii) does not occur at the 2 1 proton divalent cation stoichiometry of IE. 

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